Sitting ball



FIG. 1 is a top, front, right perspective view of a sitting ball, showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The even length broken lines in the drawings depict portions of the sitting ball that form no part of the claimed design. 

CLAIM The ornamental design for a sitting ball, as shown and described. 